The present invention relates to magneto-optic recording media for recording and reproducing under the irradiation of light and a process for producing the recording media. Particularly, the present invention relates to magneto-optic recording media comprising thin magnetic metal nitride films having perpendicular magnetic anisotropy and showing a large Faraday rotational angle and a process for producing such media.
Thin magnetic films formed on appropriate substrates (non-magnetic substrates) have been utilized as recording media (such as magnetic recording media and magneto-optic recording media). For recording media, particularly, those employed in magneto-optic recording systems (hereinafter simply referred to as magnetic-optic recording media), it has been required that they have high recording sensitivity, large magneto-optic effect (Faraday effect, Kerr effect, etc.), capability of providing large area media element with homogenous composition at a low cost, as well as excellent stability. In addition, since the magneto-optic effect becomes the greatest when the direction of the magnetization is in parallel with the propagating direction of light and the magnetization perpendicular to the plane can also satisfy the requirement for the perpendicular magnetic recording, it is also suitable to high density recording. Accordingly, those materials having magnetization perpendicular to the plane surface of medium have to be selected.
In view of such requirements, as the magnetic materials for the magneto-optic recording media, there have been used:
(1) magnetic materials employed in perpendicular magnetic recording media (typically, magnet plumbite type Ba ferrite of hexagonal close-packed (hcp) structure), or PA1 (2) MnBi, MnCuBi, MnGaGe, MnAlGe, PtCo (: polycrystalline), (YBi).sub.3 (FeGa).sub.5 O.sub.12 (: single crystal), GdCo, GdFe, TbFe, GdTbFe, TbDyFe (: amorphous), etc. PA1 the C-axis oriented thin magnetic film being formed directly or through the reflective layer on the non-magnetic support, providing that the reflective layer is formed on the thin magnetic film when the thin magnetic film is disposed directly on the non-magnetic support.
However, the Ba ferrite magnetic material (1) mentioned above has a drawback that the film formation is difficult since it requires to conduct the film formation under low substrate temperature and no large magneto-optic effect can be obtained at the wavelength region of semiconductor lasers (for example, 780 nm, 830 nm, etc.). Although there has been attempted to increase the Faraday effect by substituting trivalent iron ions with bivalent Co and tetravalent Ti, the temperature for the film formation is high in this case and remarkable restriction is imposed on the selection of the substrate material. Similar tendency is also found in other oxide thin magnetic films, for example, thin garnet films.
On the other hand, magneto-optic recording media using polycrystallize materials in (2) mentioned above involve the problem of light scattering noise by the grain boundaries and can not obtain high S/N ratio. As compared with such polycrystalline materials, those magneto-optic recording media using amorphous materials in (2) mentioned above show no such disadvantages and have been used predominantly as magneto-optic recording materials, but they involve a drawback of instability being oxidized easily.
As a result of development for those magnetic materials of excellent stability, particularly, with no oxidation in view of various disadvantages described above, iron nitrides have been noted recently. Since iron nitrides are free from rusting and ferromagnetic and, in addition, have magnetic anisotropy in the direction perpendicular to the surface of a substrate, it has been proposed to apply them as magnetic materials of high density magnetic recording media, for example, of large capacity memory devices for use in recording tapes, video tapes, computers, etc. (Japanese Patent Application Laid-Open (KOKAI) No. 55-33093, No. 59-28705, No. 60-76021, No. 61-110328, No. 62-103821, etc.).
However, such proposals for magnetic nitride materials have been made only to perpendicular magnetic recording media mainly taking notice of the perpendicular magnetic anisotropy thereof, but applications to magneto-optic recording media have not been tried yet.
Although magnetic material comprising .epsilon.-phase iron nitride Fe.sub.x N (x=2-3) is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 61-20078, the Fe.sub.x N is a magnetic powder and for forming the magnetic layer on a support, it has to be coated by using a binder, etc. Accordingly, the sensitivity of the magnetic layer tends to be lowered to provide only insufficient recording with light or heats of semi-conductor lasers, etc. and their uses are limited only to a case of practicing the thermal transfer between a master tape (Hi-Hc) and a copy tape (low-Hc), for example, in the dubbing of tapes.
Also, although magnetic recording media having a magnetic layer mainly comprising amorphous iron nitride Fe.sub.1-x N.sub.x (x.gtoreq.0.4) are disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-25204, the magnetic layer comprises an amorphous iron nitride, the amorphous iron nitride contains much nitrogen ingredient as: x.gtoreq.0.4, and the magnetic layer is not polycrystalline thin film.
Further, a production process for iron nitride magnetic material is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 62-232101, and it also discloses the C-axis orientation and perpendicular magnetic anisotropy. However, iron nitride disclosed therein has magnetic field with perpendicular magnetic anisotropy as low as about 1 KOe as shown in FIG. 3 thereof.
The present inventors have made studies from various aspects on the iron nitride magnetic materials and, as a result, it has been found that a thin magnetic film with excellent C-axis orientation having a large magnetic field with perpendicular magnetic anisotropy (H.sub.A .gtoreq.4 KOe) and a Faraday rotational angle (.theta..sub.F) of not less than 0.3 deg/.mu.m can be obtained by forming Fe nitrides films on a non-magnetic support using various film forming methods, particularly, ion beam sputtering, particularly, that the light permeability and the Faraday rotational angle can be improved by the improvement for the C-axis orientation, which is suitable to magneto-optic recording media, and further that such thin magnetic films can also be formed with other iron series element (Co, Ni) and also in nitrides containing two or more of Fe, Ni, Co. The present invention has been accomplished based on such findings.